1. Field of the Invention
The invention is directed to presence detecting systems and in particular to a cable guided detecting system and method.
2. Description of the Prior Art
One of the first leaky coaxial cable sensors is described in Canadian Patent No. 1,014,245 (Harman), entitled for a “Perimeter Surveillance System Using Leaky Coaxial Cables”, issued 19 Jul. 1977” (corresponding to U.S. Pat. No. 4,091,367). This patent describes a pulsed guided radar using one leaky coaxial cable to create an electromagnetic field and a parallel second coaxial cable to monitor the field to detect and locate intruders moving in proximity to the cables. A number of products based on this invention have been successfully deployed to detect and locate intruders at high security sites in North America and around the world.
A number of lower cost Continuous Wave (CW) leaky coaxial cable products were introduced based on contra-directionally coupled cables as described in U.S. Pat. No. 4,562,428 (Harman et al), issued Dec. 31, 1985 and co-directionally coupled cables as described in U.S. Pat. No. 4,415,885 (Mongeon) issued Nov. 15, 1983. While CW transmission used by these products reduced the cost of the intruder detectors, they are unable to locate intruders along the cable length. In practice these products are limited to buried cable, even if they use as the sensor cable a dual coaxial cable as described in U.S. Pat. No. 4,987,394 (Harman et al.), issued Jan. 22, 1991.
The contra-directionally coupled sensors described in U.S. Pat. No. 4,091,367 and No. 4,562,428 require the use of “graded” cables. One form of cable “grading” a leaky coaxial cable is to provide apertures in the outer conductor that increase in size with the length of cable. This can significantly increase the cost of the cable. One of the advantages of a co-directionally coupled sensor is that there is no need for cable “grading” since the signal path length remains constant for targets at any location along the length of the cable. In the sensor described in U.S. Pat. No. 6,577,236 the ability to locate the intruder is used to apply a different threshold for every meter of cable thereby avoiding the need for “graded” cable in this particular contra-directionally-coupled sensor. This is similar to the thresholding technique used in the system described in U.S. Pat. No. 4,091,367. The system described in this patent also uses the location information arising from the use of a coded pulse transmission to avoid the need for cable “grading”.
To date, leaky coaxial cable sensors require the use of separate transmit and receive cables. In some cases these cables are buried in separate trenches several feet apart from each other while in other cases the two coaxial lines are encased in a single jacket, commonly referred to as Siamese cables. Such Siamese cables substantially reduce the cost of installation but such cables are complex and expensive to fabricate.
The leaky cable sensor described in U.S. Pat. No. 6,577,236 (Harman), issued 10 Jun. 2003, describes a frequency modulation (FM) continuous wave (CW) leaky coaxial cable sensor with Siamese cable. The FM of the RF transmission facilitates the use of Fast Fourier Transform (FFT) digital signal processing to detect and locate intruders along the length of the cable. This technique is limited by the existence of side lobes as an artefact of the FFT. While these side lobes can be reduced by the use of windowing techniques the side lobes can cause nuisance alarms due to the relatively high attenuation of the cable. In addition these techniques attenuate the signal and can reduce the signal to noise ratio (SNR). The ability of products based on U.S. Pat. No. 6,577,236 to locate intruders along the length of the cable overcomes some of the sources of nuisance alarms with cables laying on the surface of the terrain it remains to be proven that this is adequate to make a practical rapid deployment product.
Complementary Golay codes are described in U.S. Pat. No. 5,446,446 (Harman), issued Aug. 29, 1995, and entitled “Differential Multiple Cell Reflex Cable Intrusion Detection System and Method”, which uses an acoustic cable sensor for locating intruders climbing on, or cutting through, a fence. The coded pulse is used like a Time Domain Reflectometer (TDR) to detect and locate vibrating wires inside the cable shield. Analog correlation techniques are used sequentially, to determine if a target is present in each range bin along the length of the cable. This means that for each complementary coded transmission, only one range bin is polled. Hence the update rate for each range bin is inversely proportional to the number of range bins. The present invention seeks to overcome this shortcoming by simultaneously polling all range bins to increase the update rate and thus enhance performance of the sensor.
U.S. Pat. No. 6,424,289 granted Jul. 23, 2002 describes a spread spectrum leaky coaxial cable sensor. This patent is directed at a system to locate a stationary obstacle between two leaky coaxial cables such as a rock on a railway. It is not suited to detect multiple simultaneous intruders, as is the intent of the present invention. The system described in U.S. Pat. No. 6,424,289 also claim the application of power and data over the sensor cables. The sensor described in U.S. Pat. No. 6,424,289 is not practical for the detection of multiple simultaneous targets as is required in perimeter security. The received spread spectrum response is correlated with a delayed version of the spread spectrum code to locate the obstacle. The time delay is adjusted to maximize the correlation. This works fine for very large single targets. It overlooks the fact that there will be large returns from all objects along the length of the cables and the one to be detected must be larger than any of these normal environmental returns. In the case of a human target as in an intruder detection application the response is much smaller than the normal environmental returns. In this case the movement of the human is detected using filtering as the pass band of the minute changes caused by the moving intruder is above the pass band of the normal changes in time of the environmental return.
There are two major problems associated with the use of leaky coaxial cable sensors with the cables laying on the surface of the terrain. The first is the extreme sensitivity of such sensors to minute movement of the cable relative to the surface as induced by wind or even thermal expansion of the cable. The second problem is that large objects such as trucks are often detected far beyond the desired detection zone around the cable. The present invention seeks to address these problems.
Another shortcoming is that the prior art restricts CW leaky coaxial cable sensors to one threshold per length of cable. Yet there are significant (10 to 20 dB) variations in sensitivity along a typical 100 meter length of cable due to the variations in the soil properties and installation parameters. With only one threshold these variations can cause false alarms at one location and the lack of detection of intruders at other locations. This also creates significant variations in the size of the detection zone along the length of the cable. In order to minimize these effects, the installer must adhere to a number of costly restrictions including, the use of separate cables for each burial medium (soil, concrete, asphalt etc.) and the meticulous control of cable spacing and cable burial depth. There is a need to overcome the inherent shortcomings in these products.
There is also a need to provide an intrusion detecting system that is easy to install, where the sensor cable may be buried in the terrain surface or not, and which detect intruders and locate their position with high precision and reliability.
Most current leaky coaxial cable sensors require the use of separate transmit and receive cables. In some cases these are separate cables and in other cases Siamese cables where the transmit and receive coaxial lines share a common jacket. Alternatively, either the receive or transmit cable may be replaced by an antenna to create a cable system with one antenna and one cable sensor. The present invention seeks to improve the current cable embodiments by sharing the transmit and receive function on the same leaky coaxial cable to generate significant cost savings both in hardware and in installation costs.